1. Technical Field
The present invention relates in general to an improved variable pitch propeller controller comprising a two-speed transmission between the engine and the propeller. Ideally, the controller enables the engine to run at maximum horsepower when the rotational speed (rpm) of the propeller is reduced to hold the vector sum of the aircraft forward speed, and the propeller rotational tip speed at the speed that results in the highest propeller efficiency.
2. Description of the Prior Art
U.S. Pat. No. 5,997,250 describes a propeller pitch control apparatus for an aircraft in which the desired propeller rotational speed is based on the vector sum of the aircraft forward speed and the propeller rotational tip speed, the horsepower output, and the speed of sound in the current conditions, to maximize efficiency and minimize noise. According to that invention, at high forward speeds of the aircraft, the rotation rate of the propeller should be decreased to keep the vector sum propeller tip speed at a percentage of the speed of sound based on horsepower, because this increases propeller efficiency.
Non-turbocharged reciprocating aircraft engines and turbine engines develop reduced power level at altitude because the reduced density of the air at altitude provides less oxygen. At high altitudes, such aircraft engines often must be run at maximum RPM in order to develop even this reduced power. With a fixed gear ratio between the engine and the propeller, it is not possible to maintain maximum engine power while also reducing the propeller rotation rate for best propeller efficiency at high forward speeds.
Accordingly, it is an object of the present invention to provide a simple, lightweight means whereby both the engine efficiency and the propeller efficiency can be maximized both for takeoff and for high altitude, high speed cruising.
One embodiment of a variable pitch aircraft propeller control uses a two-speed planetary gearbox between a turbine engine and an adjustable pitch propeller. For maximum efficiency, the rotation rate of the propeller is high at aircraft take-off to generate maximum static thrust. However, when at high altitude and at high speed, the propeller rotation rate is reduced to hold the vector sum of the aircraft forward speed and the propeller rotational tip speed at the speed that results in the highest efficiency for the propeller. The two-speed transmission supplies these two gear ratios. At takeoff and low altitude flight, a low gear ratio is used. At high altitude, a high gear ratio is used. The gear ratio may be manually selected by the pilot, or automatically changed by the propeller controller to obtain the best combined efficiency for the engine and the propeller.